This invention relates to processes and apparatus for producing bulk metallic glasses (bulk amorphous metals) of various desired shapes exhibiting excellent strength properties which are free from the so called cold shuts, which are the amorphous regions formed by meeting of the surfaces of the molten metal.
Various methods for producing amorphous materials have been proposed. Exemplary such methods include the method wherein a molten metal or alloy in liquid state is solidified by quenching and the resulting quenched metal (alloy) powder is compacted at a temperature below the crystallization temperature to produce a solid of the predetermined configuration having the true density; and the method wherein a molten metal or alloy is solidified by quenching to directly produce an ingot of the amorphous material having the predetermined configuration. Almost all amorphous material produced by such conventional methods had an insufficiently small mass, and it has been impossible to produce a bulk material by such conventional methods. Another attempt for producing a bulk material is solidification of the quenched powder. Such attempt, however, has so far failed to produce a satisfactory bulk material.
For example, the amorphous material produced in small mass have been produced by melt spinning, single roll method, planar flow casting and the like whereby the amorphous material in the form of thin strip (ribbon) in the size of, for example, about 200 mm in the strip width and about 30 xcexcm in the strip thickness are produced. Use of such amorphous materials for such purposes as the core material of a transformer has been attempted, but so far, most amorphous materials produced by such methods are not yet put to industrial use. The techniques that have been used for solidification forming or compaction molding the quenched powder into an amorphous material of a small mass include CIP, HIP, hot press, hot extrusion, electro-discharge plasma sintering, and the like. Such techniques, however, suffered from the problems of poor flow properties due to the minute configuration, and the problem of temperature-dependent properties, namely, incapability of increasing the temperature above the glass transition temperature. In addition, forming process involves many steps, and the solidification formed materials produced suffer from insufficient properties as a bulk material. Therefore, such methods are still insufficient.
Recently, the inventors of the present invention found that a number of ternary amorphous alloys such as Ln-Al-TM, Mg-Ln-TM, Zr-Al-TM, Hf-Al-TM and Ti-Zr-TM (wherein Ln is a lanthanide metal, and TM is a transition metal of the Groups VI to VIII) ternary systems have low critical cooling rates for glass formation of the order of 102 K/s, and can be produced in a bulk shape with thickness up to about 9 mm by using a mold casting or a high-pressure die casting method.
It has been, however, impossible to produce a large-sized amorphous alloy material of desired configuration irrespective of the production process. There is a strong needs for the development of a new solidification technique capable of producing a large-sized amorphous alloy material and an amorphous alloy having a still lower critical cooling rate for enabling the production of the amorphous metal material of larger size.
In view of such situation, the inventors of the present invention proceeded with the investigation of the bulk amorphous alloy using the ternary alloy by focusing on the effect of increasing the number of the alloy constituents each having different specific atom size as exemplified by the high glass formation ability of the ternary alloy primarily attributable to the optimal specific size distribution of the constituent atoms that are mutually different in size by more than 10%. As a consequence, the inventors found amorphous alloys of Zr-Al-Co-Ni-Cu alloy systems, Zr-Ti-Al-Ni-Cu alloy systems, Zr-Ti-Nb-Al-Ni-Cu alloy systems, and Zr-Ti-Hf-Al-Co-Ni-Cu alloy systems that have significantly lower critical cooling rates in the range of from 1 to 100 K/s, and disclosed in U.S. Pat. No. 5,740,854 (Unites States Patent corresponding to JP-A 6-249254) that alloys of Zr-Al-Ni-Cu alloy systems may be produced into a bulk amorphous alloy material with a size of up to 16 mm in diameter and 150 mm in length by quenching the melt in a quartz tube in water.
The inventors of the present invention also disclosed in U.S. Pat. No. 5,740,854 and JP-A 6-249254 that the resulting bulk amorphous alloy material has a tensile strength of as high as 1500 MPa comparable to the compressive strength and break (crack) accompanying serrated plastic flow in the tensile stress-strain curves, and that such high tensile strength and serrated plastic flow phenomenon result in excellent malleability despite the large thickness of the bulk amorphous alloy produced by casting.
On the bases of the above-described findings of the bulk amorphous alloy production, the inventors of the present invention have continued an intensive study to thereby develop a method that is capable of producing a glassy metal material of even larger size with various configurations by a simple procedure. As a consequence, the inventors proposed a process for producing metallic glass by suction casting wherein an amorphous material of large size having excellent properties can be readily produced in simple operation by instantaneously casting the molten metal material in a mold cooled with water.
Such process of metallic glass production by suction casting as disclosed in U.S. Pat. No. 5,740,854 and JP-A 6-249254 is capable of producing a columnar bulk amorphous material, and the thus produced columnar bulk amorphous material exhibits good properties. In this prior art process, however, the bottom of the water cooled crucible is moved downward at a high speed and the molten metal is instantaneously cast into a vertically extending water-cooled mold to thereby attain a high moving speed of the molten metal and a high quenching rate.
In such production process, the molten metal is fluidized with the surface of the molten metal becoming wavy, and the surface area of the molten metal is increased with the increased surface area contacting the outer atmosphere. In some extreme cases, the molten metal is fluidized into small separate bulk molten metal droplets before being cast into the vertically extending mold. Therefore, the surfaces of the molten metal often meet with each other upon casting of the molten metal into the vertically extending water-cooled mold, and the so called cold shuts or discontinuities are formed at the interfaces of the thus met interfaces. The resulting bulk amorphous material thus suffered from inferior properties at such cold shuts, and hence, the bulk amorphous material as a whole suffered from poor properties.
In addition, the metal material is melted in a water-cooled hearth, and the part of the metal in contact with the hearth is at a temperature below the melting point of the metal material even if the metal material is in molten state. The part in contact with the hearth, therefore, is likely to induce nonuniform nucleation. In the above-described suction casting, such part of the molten metal which may induce uniform nucleation is also cast into the vertically extending water-cooled mold and there is a fair risk of crystal nucleus formation in the corresponding part.
Furthermore, since the bottom of the water-cooled crucible is moved downward at a high speed, the process suffered from a fair chance of the molten metal entering into the gaps formed between moveable parts and the like to reduce the reproducibility. In some cases, the molten material entered is even caught in such gaps to result in failure, stop, or incapability of operation.
An object of the present invention is to obviate the drawbacks of the above-described techniques and to provide processes and apparatus for producing a metallic glass which is free from the so called cold shuts which are formed by amorphousizing at the interfaces where the surfaces of the molten metal cooled to a temperature below the melting point by contact with outer atmosphere have met; and which is also free from crystalline part where crystal nuclei have developed through nonuniform nucleation by the molten metal below its melting temperature. In other words, an object of the present invention is to provide a simple process and a simple apparatus for producing a metallic glass which are capable of producing a bulk metallic glass of desired shape exhibiting excellent strength properties in a simple procedure at a high reproducibility by selectively cooling the molten metal above its melting temperature at a rate above the critical cooling rate.
To attain such object, there is provided by the present invention a process for producing a bulk metallic glass of desired shape comprising the steps of:
filling a metal material in a hearth;
melting said metal material by using a high-energy heat source which is capable of melting said metal material;
pressing a molten metal at a temperature above the melting point of said metal material to deform the molten metal at a temperature above the melting point into the desired shape by at least one of compressive stress and shear stress, while avoiding surfaces of the molten metal cooled to a temperature below the melting point of said metal material from meeting with each other during the pressing; and
cooling said molten metal at a cooling rate higher than the critical cooling rate of the metal material simultaneously with or after said deformation to produce the bulk metallic glass of desired form.
According to the present invention, there is also provided by a process for producing a bulk metallic glass wherein said molten metal at a temperature above the melting point of said metal material is pressed while avoiding not only the meeting of the surfaces of the molten metal cooled to a temperature below the melting point of said metal material with each other but also meeting of such molten metal surface with another surface cooled to a temperature below the melting point of said metal material.
In this process, the pressing and deforming of said molten metal is preferably accomplished by selectively rolling said molten metal at a temperature above the melting point of said metal material into the plate shape or other desired shape with a cooled roll for rolling.
Preferably, after melting said metal material filled in the hearth, the molten metal at a temperature above the melting point rising over the hearth is selectively rolled with simultaneous cooling by rotating said cooled roll and moving the hearth in relation to said high energy heat source and said rotating cooled-roll to thereby produce a metallic glass of plate shape or other desired shape.
It is also preferable to use a hearth of an elongated shape, and the melting, rolling of the molten metal at a temperature above the melting point, and the cooling are continuously conducted by using such hearth of an elongated shape and moving such hearth in relation to said high energy heat source and said rotating cooled roll to thereby continuously produce a metallic glass of elongated shape or other desired shape.
The cooled roll for rolling is preferably provided at the position corresponding the hearth with a molten metal-discharging mechanism for discharging the molten metal at a temperature higher than the melting point from the hearth, said molten metal-discharging mechanism being fabricated from a material of low thermal conductivity.
It is also preferable to accomplish the pressing and deforming of said molten metal by selectively transferring said molten metal at a temperature above the melting point of said metal material into a cavity of the desired shape in the mold provided near said hearth without fluidizing the molten metal, and pressing the molten metal with a cooled upper mold without delay to forge the molten metal into the desired shape together with simultaneous cooling.
In this case, after melting said metal material filled in the hearth, said hearth and said lower mold is preferably moved to right underneath said upper mold and the upper mold is descended toward said lower mold without delay to thereby selectively transfer the molten metal at a temperature above the melting point into said mold where it is pressed and cooled to produce the metallic glass of desired shape by forging.
To attain the above-described object, there is provided by the present invention an apparatus for producing a metallic glass comprising a hearth for accommodating a metal material, a means for melting said metal material in said hearth, a means for pressing a molten metal which has been melted by said metal material-melting means at a temperature higher than the melting temperature to deform the molten metal into the desired shape by at least one of compressive stress and shear stress, while avoiding the surfaces of the molten metal cooled to a temperature below the melting point of said metal material from meeting with each other during the pressing; and a means for cooling said molten metal at a cooling rate higher than the critical cooling rate of the metal material simultaneously with or after said deformation by said pressing means.
In this apparatus, said molten metal is preferably pressed while avoiding not only the meeting of the surfaces of the molten metal cooled to a temperature below the melting point of said metal material with each other but also meeting of such molten metal surface with another surface cooled to a temperature below the melting point of said metal material.
Preferably, said pressing means doubles as said cooling means.
Preferably, said pressing means has a cooled roll for rolling and a mold provided near said hearth.
Preferably, the molten metal at a temperature above the melting point rising over the hearth is cast into said mold by said cooled roll by rotating said cooled roll and moving said hearth and said mold in relation to said cooled roll and said melting means to accomplish the rolling by said cooled roll and said mold.
Preferably, said hearth is of elongated shape, and the rolling and the cooling by said cooled roll and said mold is continuously conducted by moving said hearth and said mold in relation to said cooled roll and said melting means.
Preferably, said cooled roll for rolling is provided at the position corresponding said hearth with a molten metal-discharging mechanism for discharging the molten metal at a temperature higher than the melting point from the hearth, said molten metal-discharging mechanism being fabricated from a material having low thermal conductivity.
Preferably, said pressing means has a lower mold provided near said hearth into which the molten metal discharged from said hearth is filled, and an upper mold which forges the molten metal filled in said lower mold together with said lower mold.
Preferably, after melting said metal material filled in the hearth, said hearth and said lower mold are moved in relation to said melting means and said upper mold until said upper mold is positioned at a position opposing said hearth and said lower mold, and the upper mold is descended or the lower mold is ascended without delay to thereby transfer the molten metal from said hearth into said mold where it is forged.
Preferably, said upper mold is provided at the position corresponding said hearth with a molten metal-discharging mechanism for discharging the molten metal at a temperature higher than the melting point from the hearth, said molten metal-discharging mechanism being fabricated from a material having low thermal conductivity.
The upper mold is preferably provided at the position corresponding the hearth with a molten metal-discharging mechanism for discharging the molten metal at a temperature higher than the melting point from the hearth, said molten metal-discharging mechanism being fabricated from a material of low thermal conductivity.
In the present invention, the phrase xe2x80x9cmeetingxe2x80x9d of xe2x80x9cthe surfaces cooledxe2x80x9d means the xe2x80x9cmeetingxe2x80x9d of xe2x80x9cthe surfaces of the molten metal cooled to a temperature below the melting point of said metal materialxe2x80x9d in a narrower sense. In a broader sense, this phrase also include the case wherein xe2x80x9cthe surfaces of the molten metal cooled to a temperature below the melting point of said metal materialxe2x80x9d meet with xe2x80x9cother surfaces cooled to a temperature below the melting point of said metal materialxe2x80x9d such as the surface of the hearth cooled by water. It should be noted that the phrase xe2x80x9cthe surfaces of the molten metal cooled to a temperature below the melting point of said metal materialxe2x80x9d are the surfaces of the molten metal cooled to a temperature below the melting point by contact with outer atmosphere, mold, hearth or the like.
The phrase xe2x80x9cpressing a molten metal at a temperature above the melting point of said metal material to deform the molten metal, while avoiding the surfaces cooled to a temperature below the melting point of said metal material from meeting with each other during the pressingxe2x80x9d used herein does not only mean the pouring of the molten metal maintained at a temperature above the melting point from the cooled hearth into the mold followed by pressing, while avoiding the formation of cold shuts which are formed by the meeting of the surfaces cooled to a temperature below the melting point of said metal material caused by fluidization or surface wave-formation. This phrase also includes use of a mold fabricated from a material such as quartz which is not thermally damaged at a temperature above the melting point of the metal material, and heating of the lower mold to a temperature near the melting point, preferably, to a temperature above the melting point, followed by pouring of the metal molten with a high energy source, for example, a radio frequency heat source and maintained at a temperature above the melting point into the preliminarily heated lower mold without forming any surface which is cooled to a temperature below the melting point; and pressing with the cooled upper mold to thereby conduct the pressing and quenching at a rate above the critical cooling rate. Namely, if the metal material used is a material with an extremely low critical cooling rate, the metal molten in a quartz tube may be directly poured and cooled in water while maintaining its shape.
In other words, the cold shuts are formed when the pressing, deformation, compression, shearing of the molten metal are not conducted at a rate higher than the critical cooling rate and meeting of the cooled surface are not avoided. When a metal having a certain critical cooling rate, for example, 10xc2x0 C./sec is used, an amorphous bulk material without cold shuts can be produced only when the time between the molten state and the deformation and the decrease in temperature attain the predetermined critical cooling rate (higher than 10xc2x0 C./sec in this case); and the meeting of the cooled surface is avoided.
The term xe2x80x9cdesired shapexe2x80x9d used herein is not limited to any particular shape as long as the metallic glass material is formed through pressing or forging by using an upper press roll or forging mold of various contour and a lower press surface or forging mold of various contour which are controlled and cooled in synchronism. Exemplary shapes include, a plate, an unspecified profile plate, a cylindrical rod, a rectangular rod, and an unspecified profile rod.